The present invention relates to in-situ fluorometers of the type adapted for underwater use, and especially deep underwater use, and constitutes an improvement relative to, for example, the fluorometer disclosed in commonly owned U.S. Pat. No. 3,666,945 of Frank Frungel et al.
The basic principle of operation of deepsea in-situ fluorometers is relatively simple. The fluorometer has a power supply and various meters, and the like, provided onboard a vessel. A cable, having a length for example on the order of 1000 meters, is connected to the power supply and meters, and hangs down overboard, with the fluorometer's measuring equipment connected at its other end, e.g., 1000 meters below the surface, the fluorometer sometimes being towed. Fluorometers of the type in question can alternatively be used in less deep rivers and bodies of water, i.e., despite their heavy-duty deepsea construction.
The underwater equipment includes a light transmitter and a light receiver. Pulses of light emitted from the transmitter are used to excite fluorescent substances in the water being investigated, e.g., bioplasma already present in the water and being investigated to ascertain water composition, or fluorescent tracer elements added to the water for example when investigating the movement or flow of water within a body of water. The emitted radiation excites the fluorescent material. The fluorescent radiation is detected by the receiver and converted into an electrical signal, which is then amplified and/or otherwise processed and transmitted along the length of the long cable to onboard the vessel, where the signals are fed into meters, or the like, for read-out.
Although the basic operating principle is simple, practical problems with such in-situ fluoremeters are so enormous as to greatly affect operativeness itself.
Thus, for example, it is conventional practice to utilize for the onboard power supply a stabilized voltage source. However, the resistance of the e.g., 1000-meter cable connecting the voltage source to the underwater equipment is quite substantial, requiring the provision of a matching resistance, to assure that the voltage actually fed to the underwater equipment is of sufficiently large magnitude. This is inherently power-consumptive. Even if the power supply itself is voltage-stabilized, it is also necessary that the voltage received by the underwater equipment be stabilized, for the obvious reason of measuring accuracy. If different lengths of connecting cable are employed, for different depth ranges to be explored, this will either have to be taken into account by corresponding adjustment of the underwater equipment and/or will involve further power waste. Despite all this, it has been found in practice that fluctuations in the voltage supplied to the underwater equipment are simply unavoidable. If the voltage magnitude employed is to be kept reasonably low, such fluctuations can seriously detract from measuring accuracy.
A further problem relates to the conflicting requirements of high sensitivity and immunity to the effect of daylight penetrating below water. In the past, the photosensitive element of the underwater receiver was typically a photomultiplier, to take advantage of the high photosignal strengths which such elements could achieve. However, if underwater daylight impinged on such photomultipliers, a corresponding D.C. current component was generated exhibiting the same high gain as the actual pulsed light signal of interest, e.g., a gain factor of 10.sup.6. This resulted in very severe electronic noise and also in thermal overloading of the photomultipliers. Accordingly, it was necessary, in such earlier fluorometers, either to work in darkness or else somehow shield the photomultiplier from underwater light, by relatively complicated means. For that reason, commonly owned U.S. Pat. No. 3,666,945 proposed replacement of the photomultiplier with a simple silicon semiconductor photodiode. Although photodiodes do not exhibit the high gain of photomultipliers, their quantum efficiency per se is higher than the primary efficiency of a photomultiplier. This made it possible to avoid the enormous problems of exposure to underwater light, while still generating a signal of sufficient strength for processing.
This improvement, however, was not without problems. Exposure of a simple photodiode to underwater light also results in the generation of a great amount of singal noise, which may indeed be quite violent and energetic. Furthermore, this noise does not just consist of a steady D.C. component corresponding to steady underwater illumination; such a component can be suppressed, more or less, using high-frequency filtering. Rather, the noise generated is to a great extent high-frequency noise resulting from electronic excitation mechanisms internal to the structure of the photodiode element, and this noise cannot be suppressed by merely using high-frequency filters. This high-frequency noise forms a background for the signal of interest, and therefore the signal of interest must somehow be made sufficiently strong to be distinguished from its background. For example, when investigating water movements using fluorescent tracers, the requisite boost in the level of the signal of interest is achieved by increasing the amount of fluorescent tracer employed. However, fluorescent tracers are quite expensive. For example, the investigation of the movement of a large body of water, e.g., the Gulf Stream, requires many hundreds of runs, and the amount of tracer needed for each run may cost on the order of $100. The cost of the tracer, accordingly, can in the long run dwarf the cost of all the equipment itself. If, for example, the sensitivity of the receiver could be increased by 3--i.e., if one could use 1/3 the normal amount of tracer to achieve the same signal strength relative to its background noise--this would already signify a very great cost saving.
A problem which, as will become clearer below, is interconnected with sensitivity is the manner in which the signal to be processed is formed. In the prior art, the peak value of the voltage produced by the photodiode is ascertained, and thereafter this peak-voltage value is used for signal processing. The prior-art peak-voltage technique has been adopted as a logical consequence of adopting a pulse technique. Pulse techniques have been adopted to perform intermittent, relatively high-energy measurements, and thereby enormously reduce power consumption, compared to what would be the case if, unthinkably, an uninterruptedly constant fluorescence-excitation technique were employed. Inasmuch as pulse techniques make possible the generation of high peak values, it has thus far seemed only natural to base the signal processing on the peak-voltage value produced by the receiver's photodiode. However, this can create serious practical difficulties with respect to accuracy. When the light source used for the underwater transmitter unit of the apparatus is for example a flash discharge lamp, we have found that the quantity of light emitted per light pulse is quite invariable, if the transmitter is well enough designed. In contrast to the total light emitted per light pulse, however, the peak value of light emitted per light pulse has been found to be variable enough to detract from measuring accuracy. Thus, prior-art systems, basing their signal processing as they do on the detected peak-voltage value of the photodiode, are limited as to accuracy in correspondence to the variation in the peak-value of emitted light per pulse. Loss of accuracy is a disadvantage whose significance requires no explanation. However, a related disadvantage results from the peak-voltage technique of the prior art. In the type of fluorometers in question, the output signal transmitted via cable to the onboard equipment is logarithmic; i.e., the photodiode's peak-voltage, or an amplified voltage derived therefrom, is applied to the input of a log amplifier, and the log amplifier's output signal is transmitted to the onboard equipment. Unfortunately, with the type of signal-generation and-processing techniques traditional for such fluorometers, the linear range of the logarithmic transformation achieved is found to be limited to about three decades, and to be furthermore considerably less than ideal. Accordingly, the signal transmitted to the onboard equipment will often be imperfectly logarithmic and for that reason actually inaccurate; very complex techniques, e.g., involving computer analysis, may be required to correct the imperfectly logarithmic character of the signal received onboard, to convert it into a sufficiently accurate signal. Alternatively or in addition thereto, because of the limited number of decades through which near-ideal logarithmization can be implemented, it may be necessary to change the decades-range of the log amplifier, when the onboard personnel observe that the returned signal is of a value not corresponding to the decades-range at which the logarithmization can properly occur. This may require hoisting of the underwater apparatus onboard to readjust the log amplifier, or complex techniques such as remote-control adjustment of the log amplifier's range.
A still further problem with the type of insitu fluorometer in question relates to component stability. Even when the components of the underwater apparatus, and especially the receiver, are tested and selected with utmost care, e.g., integrated circuits selected in accordance with military specifications, instability in the null point of the measuring system has been unavoidable. Even highest-quality semiconductor elements exhibit only limited stability in the course of time, obviously due to internally occurring diffusion processes in silicon. Of course, lack of stability is always undesirable. However, in fluorometers of the type in question, the practical difficulties are already so numerous, interrelated and connected with operativeness and accuracy per se, that lack of null-point stability, particularly in combination with all the problems of sensitivity, accuracy, limited power consumption, and so forth, becomes disproportionately troublesome compared to many other types of measuring instruments.
The problems with prior-art fluorometers as discussed above, are very difficult to attack individually, because of their interrelated nature. Attempts to alleviate just one of these problems will typically worsen others, or bring other problems into existence. Likewise, a mere willingness to employ prior-art principles, but with no limits on the cost of the circuitry used to implement them, in the hope of avoiding the problems discussed above by means of ultra-high-quality components, and the like, are not per se sufficient, because these problems flirt with the limits of what is commercially available at any price. Furthermore, of course, unlimited cost is not actually acceptable.